1. Field of Invention
The present invention relates to an optical information recording method for recording information to an optical information recording medium with which data is optically recorded and reproduced, and to an apparatus for recording the same, and a recording medium, and more particularly relates to a method for recording information to an optical information recording medium with which information is recorded at a plurality of different linear velocities.
2. Description of the Related Art
Optical disks, optical cards, optical tapes, and so forth have been proposed and developed in recent years as media for optically recording data. Optical disks have become especially popular as media that allow large volumes of data to be recorded and reproduced at high density.
For instance, in the case of a phase-change optical disk, data is recorded and reproduced by the following method. The recording film of an optical disk is irradiated with a laser beam focused by an optical head and stronger in power than the reproduction power (this power level will be referred to as the recording power, and expressed as Pp), and when the temperature of the recording film goes over the melting point, the molten portion is rapidly cooled as the laser beam is transmitted, and this forms a mark in an amorphous state. When the recording film is irradiated with a laser beam that has been focused enough to raise the temperature of the recording film to at least its crystallization temperature but below its melting point (this power level will be referred to as the erasure power, and expressed by Pb), the irradiated portion of the recording film enters a crystalline state. As a result, a recorded pattern consisting of marks (amorphous regions) and spaces (crystalline regions) corresponding to a data signal is formed on the optical disk. The data is then reproduced by taking advantage of the difference in reflectance between the crystalline and amorphous regions.
As described above, to form marks on a medium, it is necessary to modulate the power level of the emitted laser beam at least between an erasure power and a recording power. The pulse waveform used in the modulation operation is called a recording pulse. Many recording methods for forming a single mark with a plurality of recording pulses have already been disclosed. The plurality of recording pulses is called a recording pulse train. FIG. 10a shows an example of a recording pulse train. A pulse at the front part of the recording pulse train is called the leading pulse 501, a pulse at the end of the recording pulse train is referred to as the trailing pulse 503, and a pulse between the leading pulse 501 and the trailing pulse 503 is called a multi-pulse 502. The number of recording pulses that make up the recording pulse train varies with the recording code length (that is, the length of the recording code with respect to a channel clock period Tw), and at the shortest code length, there may be only one recording pulse. The intensity of the laser beam is modulated as shown in FIG. 10b on the basis of this recording pulse train. As a result, a mark 302 is formed on a track 301 as shown in FIG. 10c. A method in which a mark is formed by using a laser irradiation waveform in which the pulse level is varied between the leading portion and the trailing portion as shown in FIG. 11, instead of using a recording pulse train, has also been disclosed.
At present, DVDs and other such optical information recording media primarily make use of constant linear velocity (CLV) recording. This is a system for recording data at substantially the same linear velocity, transfer rate, and linear density over the entire surface of the medium. An advantage therefore is that there is no change in the laser beam irradiation conditions or in heating/cooling conditions during recording and reproduction. On the other hand, since the rotational speed of the medium varies with the recording or reproduction position (that is, the radial position) on the medium, controlling the rotational speed changes of a spindle motor is essential.
In contrast, a constant angular velocity (CAV) recording system, in which the rotational speed and the linear density of the medium are kept substantially constant over the entire surface of the medium, has been proposed. Unlike with a CLV recording system, a CAV recording system does not require control of the rotational speed changes of a spindle motor for rotating the medium, so an advantage is that the spindle motor and the control circuit thereof can be produced at lower cost. Also, there is no need to wait for a recording or reproduction operation until a specific rotational speed is attained after seeking the recording or reproduction position, so the access speed with respect to the medium can be shortened. On the other hand, a CAV recording system, the linear velocity and the transfer rate of the medium vary with the recording or reproduction position in the medium. Therefore, the conditions under which the medium is irradiated with the laser beam, and the heating/cooling conditions vary with the recording or reproduction position.
Improving recorded signal quality in either of these systems is important in terms of recording or reproducing large volumes of data at high density. Accordingly, methods have already been disclosed for improving signal quality by adjusting the recording power with respect to a certain linear velocity (the recording or reproduction position in the case of CAV recording). One such method that has been disclosed involves recording a test signal while the recording power or the erasure power is varied, and determining the power levels on the basis of the recorded test signal so that the asymmetry or degree of modulation of the reproduction signal is optimized (see Patent Document 1, for example).
However, with the above-mentioned conventional recording method, if the linear velocity is varied over a large range in CAV recording, the optimal conditions will be different for each linear velocity, so a data signal cannot be recorded stably and with good signal quality. This problem will now be described.
During recording at a high linear velocity, the spindle motor rotates at high speed. Consequently, if there is even a slight axial runout or eccentricity to the disk, this will have a major impact on servo operation. Specifically, if there is axial runout in the disk, the actuator of the optical head will oscillate strongly in the optical axis direction. If there is eccentricity in the disk, the actuator will oscillate strongly in the direction parallel to the plane of the disk. If this oscillation exceeds the response characteristics of the actuator, the actuator will be unable to keep up with the axial runout or eccentricity of the disk, defocusing or off-tracking will occur, and the signal cannot be recorded stably.
On the other hand, during recording at a low linear velocity, the relative speed between the laser spot and the medium is lower, and cooling after melting caused by laser irradiation is slower, so recrystallization proceeds from the edges of the molten part, and the remaining portion becomes a mark. Accordingly, as shown in FIG. 12, a molten region 303 is larger than a mark 302, and the end of the molten region extends all the way to the walls 304 of a track 301 (that is, a guide groove or land). As a result, this affects the fine shape of the walls 304 (causing looseness), recrystallization does not occur in part of the molten region reaching the walls 304, and parts of the mark stick to the walls 304. Consequently, the shape of the mark is distorted and this lowers the quality of the reproduction signal.    Patent Document 1: Japanese Unexamined Patent Publication No. H10-64064.